Cropping Off the Excess In a Masked Object
To create a Clipping Mask, make sure the object you’d like to become the Mask is in front. Select both this object as well as all objects to be Masked, and go to Object > Clipping Mask > Make. Easy!
One of the problems in working with Clipping Masks is that it’s easy to inadvertently select an object that you can’t see (outside the Mask). One solution to this problem (once you are totally satisfied with your Mask and know you won’t have to edit it) is to use the Mask to Crop the rest of the objects. Select the Masked objects and the Mask, and in the Pathfinder palette (Window > Pathfinder), click the forth button from the left in the Pathfinder section. Anything that the Mask was hiding will be cropped away.
Tip provided by Jeff Witchel, Certified Adobe® Training Provider.